This application relates to an automatic power control circuit for a laser driver.
Referring to FIG. 1, in a conventional laser diode driver, which may be used for driving a laser diode in response to an electrical data signal for launching a corresponding optical data signal into an optical fiber, the laser diode D10 is connected between a positive supply rail +V and a node 14. Current is supplied to the node 14 by a transistor Q1, having its collector connected to the node 14 and its emitter connected to ground. An average power control signal V.sub.ref is applied to the non-inverting input of a differential integrating amplifier 18, whose output is connected to the base of the transistor Q1 through a loop filter composed of a resistor 22 and a capacitor 26. A monitor or feedback photodiode D30, which receives a small proportion of the optical energy emitted by the laser diode, is connected between the positive supply rail +V and a node 50, which is connected to ground through a resistor 38. The potential at the node 50 depends on the power output of the laser diode D10. The node 50 is also connected to the inverting input of the differential amplifier 18.
The incoming electrical data signal is applied to the base of a transistor Q2 through an inverter 40 and is applied directly to the base of the transistor Q3. The emitters of the transistors Q2 and Q3 are connected to a constant current source 42. The collector of the transistor Q2 is connected to the node 14 and the collector of the transistor Q3 is connected to the positive supply rail +V. When the data signal is high, the transistor Q2 is turned off and the transistor Q3 is turned on, and accordingly the current source 42 supplies no current to the laser diode D10, whereas when the data signal is low, the transistor Q2 is on and the transistor Q3 is off and the current source 42 supplies a relatively high current to the laser diode D10.
It is not desirable that current supplied by the transistor Q1 should be so low that when the data signal is high, and the transistor Q2 is off, the laser diode D10 should turn off, since operation in this mode adversely affects the rise and fall times of the optical signal generated by the laser diode. Conversely, if the current supplied to the laser diode D10 is too high when the transistor Q2 is on, the useful life of the laser diode is reduced.
The monitor diode D30, differential amplifier 18 and transistor Q1 operate as an automatic power control (APC) loop, controlling the average power level at which the laser diode D10 operates. Typically, the voltage of the signal V.sub.ref is selected so that the average power level in the presence of modulating data is about 50 percent of the maximum power at which the laser diode can operate without significantly reducing its useful life, and when the data signal is high, the laser diode operates at about 10 percent of its maximum power level whereas when the data signal is low, the laser diode operates at about 90 percent of its maximum level. This achieves a sufficient modulation depth to allow the data to be recovered at the receiving end of the fiber optic cable while ensuring that the laser diode does not turn off and is not overdriven.
The APC loop is characterized by a frequency response curve having a corner frequency which defines the maximum frequency at which energy coupled to the loop will influence operation of the laser diode D10. Energy at frequencies above the corner frequency will not affect the operation of the APC loop.
Energy at the frequency of the data signal is coupled into the APC loop by the laser diode D10 but operation of the APC loop should be independent of the frequency content of the data signal. Accordingly, the corner frequency of the loop should be below the minimum frequency present in the data signal. This condition is satisfied if the data signal has zero DC content and the runs of consecutive 1's and 0's are short.
In general, digital source data that is to be propagated over a channel is composed of multi-bit words, which are coded as a serial binary data stream for serial propagation.
When serial binary data is propagated over a channel, it is desirable that the baseband data have zero DC content. Many channel codes, such as Manchester and bi-phase codes, provide a baseband data stream which has zero DC content. When the channel code is selected so that the baseband data has zero DC content, the condition for proper operation of the APC loop is met provided that the runs of 1's and 0's are short.
The SMPTE 259 and SMPTE 292 standards for serial digital interface signals each prescribe a standard for mapping video data in the form of 10-bit words to a serial binary data stream. SMPTE 259 and SMPTE 292 each prescribe a polynomial, or PN, scrambler which functions well to generate baseband data having minimal DC content provided that the video data supplied to the scrambler is random, or nearly random. When the source of the video data is a camera, noise generated in the camera provides a sufficient degree of randomness. However, the content of some computer generated video data is not sufficiently random and the PN scrambler can generate baseband data having very long runs of consecutive 1's and 0's in response to these so-called pathological signals. These long runs of 1's and 0's generate a significant frequency content below the desired corner frequency of the conventional APC loop. In the case of a pathological signal, the DC content of the baseband data causes a typical automatic power control loop to force the average power level upwards or downwards in inverse proportion to the magnitude of the DC content of the signal. In the case of positive DC content, the average power output is reduced allowing the possibility of turning the laser off. In the case of negative DC content, the average power output is increased allowing the possibility of over-driving the laser. Even if these two extremes are not encountered, this data dependent modulation of the average power level is seen at the receiver as additional amplitude modulation which subtracts from the dynamic range of receiver circuitry, reducing the transmission loss budget and leading to difficulty in recovering the data at the receiving end of the optical fiber cable.
While it is theoretically possible to choose an APC loop with its corner frequency sufficiently low that it is transparent to the pathological signal, i.e. the pathological signal does not affect operation of the loop, this is not done in practice. The corner frequency would have to be as low as 10-100 Hz, depending upon the efficiency of the laser diode/feedback diode pair, in order to pass a SMPTE 259 or SMPTE 292 pathological signal. Unfortunately, the loop would then be too slow to compensate for component offset drifts due to thermal variations. These variations, left unchecked, generate similar amplitude modulation phenomena to the pathological signal, but because the variations are a strong function of temperature, even a short run of consecutive 1's and 0's may cause distortion. Therefore, based on laser physics and typical data rates, the APC corner frequency is usually set to approximately 1-4 kHz.
This invention allows the corner frequency of the APC loop to remain high, thus compensating for thermal dependent distortion, as well as long term laser aging, without over compensating for long strings of 1's and 0's.